Reversing Evolution

When it comes to plants and animals, biologists think of DNA as the sole storehouse of genetic information. A surprising new study challenges that notion. The mustard plant Arabidopsis thaliana sometimes ends up with its grandparents' good copy of a gene instead of the mutant ones belonging to its parents. So researchers are putting forth the radical proposal that plants contain an inheritable cache of RNA that can reverse evolution, undoing mutations and restoring a gene to its former glory.

Susan Lolle and Robert Pruitt of Purdue University in West Lafayette, Indiana first discovered that genes could go back in time about 3 years ago while studying a gene in A. thaliana called HOTHEAD. The surprise involved the reversal of a mutation. In plants with both copies of HOTHEAD mutated, the floral parts are all stuck together into a little ball. When such a plant self-fertilizes, its progeny inherit two copies of the mutated gene and all of them should have balls instead of flowers.

Instead, Lolle and Pruitt found that 1% to 10% of the offspring produced normal flowers, indicating that at least one copy of the mutant gene had reverted to the nonmutated wild-type. "It's something that Mendelian genetics has not prepared us for," says Pruitt. He and Lolle report the finding in the 24 March issue of Nature. Pruitt says RNA "templates" derived from the original gene and stored in the gametes are the best candidates for reverting the mutant gene to its original state.

The paper "suggests the existence of a unique genetic memory system that can be invoked at will," says Vincent Colot of the Plant Genomics Research Unit at Genopole in Evry, France. If confirmed and extended to animals, the new findings could profoundly affect biomedicine as well as population genetics. For example, geneticists trying to assess disease risk would have to take into consideration the makeup of this RNA memory, notes Emma Whitelaw of the University of Sydney, Australia.